1. Technical Field
The present invention relates to a power-generating device, a secondary cell, and an electronic apparatus.
2. Related Art
When a piezoelectric material such as lead zirconium titanate (PZT), quartz crystal (SiO2), or zinc oxide (ZnO) is deformed in response to an external force, electrical polarization is induced inside the material, and positive and negative charges appear on the surface. Such a phenomenon is called a piezoelectric effect. There has been proposed an electrical power generation method of vibrating a cantilever to thereby make a weight repeatedly act on the piezoelectric material, and thus taking out the charge generated on the surface of the piezoelectric material as a current using such a characteristic the piezoelectric material has.
For example, by vibrating a metal cantilever having a mass disposed at the tip and a thin plate made of the piezoelectric material bonded thereto, and taking out the positive and negative charges generated on the piezoelectric material due to the vibration, an alternating current is generated. Then the alternating current is rectified by diodes, and then stored in a capacitor, and then taken out as electricity. Such a technology has been proposed in, for example, JP-A-7-107752. Further, there has been also proposed a technology of arranging that a junction is closed only in the period during which the positive charges are generated in a piezoelectric element to thereby make it possible to obtain a direct current without causing a voltage loss in the diodes (see, e.g., JP-A-2005-312269). By using these technologies, downsizing of the power generation unit can be achieved. Therefore, an application such as incorporation of the power generation unit to a small-sized electronic component instead of a battery is expected.
However, in the proposed power generation unit according to the related art, there arises a problem that the obtainable voltage is limited up to the voltage generated by the electrical polarization of the piezoelectric material. Therefore, in most cases, an additional step-up circuit is required, and there arises a problem that it is difficult to sufficiently downsize the power generation unit. Further, an electrical power is usually required to drive the step-up circuit, and there arises a problem that the step-up operation becomes difficult if the electrical energy in the capacitor is reduced. Although it is also possible to dispose a full bridge rectifier or a voltage doubler rectifier in parallel to the step-up circuit in order for solving this problem, there arises a problem that the power generation unit grows in size.